Erythropoietin (EPO) is a glycoprotein hormone that is normally synthesized and secreted by peritubular cells in the kidney and functions as the principle homeostatic regulator of red blood cell production. Recombinant human erythropoietin (rHuEPO) is used clinically to treat anemias and increase red blood cell production in numerous different conditions, such as perisurgery, chronic renal failure, side effects of HIV or HCV treatment, and side effects of cancer chemotherapy. Pharmaceutical biosynthesis of glycoproteins such as EPO is complicated by the need for both high levels of expression and appropriate posttranslational processing, which involves the addition of N-linked and O-linked branched oligosaccharide chains.
In glycoproteins, sugars are attached either to the amide nitrogen atom in the side chain of asparagine (termed an N-linkage) or to the oxygen atom in the side chain of serine or threonine (termed an O-linkage). The process for forming N-linked carbohydrates begins with the addition of 14 monosaccharides to a lipid-linked dichol in the endoplasmic reticulum (ER). After its formation, this carbohydrate complex is then transferred to the protein by the oligosaccharyltransferase (OST) complex in a process termed “core glycosylation” in the ER. The oligosaccharyltransferase (OST) complex is a multi-protein unit comprised of ribophorin I, II, OST48 and DAD1 (Kelleher and Gilmore 1997 PNAS 94(10):4994-4999; Kelleher et al. 2003 Molecular Cell 12(1):101-111; Kelleher et al. 1992 Cell 69(1):55-65).
Subsequently, the polypeptides are transported to the Golgi complex, where the O-linked sugar chains are added and the N-linked sugar chains are modified in many different ways. In the cis and medial compartments of the Golgi complex, the original 14-saccharide N-linked complex may be trimmed through removal of mannose (Man) residues and elongated through addition of N-acetylglucosamine (GlcNac) and/or fucose (Fuc) residues. The various forms of N-linked carbohydrates have in common a pentasaccharide core consisting of three mannose and two N-acetylglucosamine residues. Finally, in the trans Golgi, other GlcNac residues can be added, followed by galactose (Gal) and a terminal sialic acid (Sial). Carbohydrate processing in the Golgi complex is called “terminal glycosylation” to distinguish it from core glycosylation.
Sialic acid is a generic name for a family of about 30 naturally occurring acidic monosaccharides that are frequently the terminal sugars of carbohydrates found on glycoproteins and glycolipids. Sialylation of recombinant glycoproteins is very important and may impart many significant properties to the glycoprotein including charge, immunogenicity, resistance to protease degradation, plasma clearance rate, and bioactivity.
The final complex carbohydrate units can take on many forms, some of which have two, three or four branches (termed biantennary, triantennary or tetraantennary). An exemplary N-linked biantennary structure is shown below:

A number of enzymes involved in glycosylation utilize divalent cations as co-factors. For example, numerous enzymes involved in the dolichol-linked oligosaccharide synthesis require divalent cations as co-factors for activity (Couto et al. 1984 J. Biol. Chem. 259(1):378-382; Jensen and Schutzbach 1981 J. Biol. Chem. 256(24):12899-12904; Sharma et al. 1982 European Journal of Biochemistry 126(2):319-25). The enzyme which catalyses the addition of O-linked carbohydrate to the polypeptide also requires a divalent cation for activity (Sugiura et al. 1982 J. Biol. Chem. 257(16):9501-9507). Manganese (Mn++) is a required co-factor for the enzyme β-galactoside-α-1,3-galactosyltransferase, which catalyzes the addition of terminal galactose to elongating N-acetyl-glucosamine sugars (Witsell et al. 1990 J. Biol. Chem. 265(26):15731-7). It was previously reported that manganese at a concentration of 0.1 mM or 1 mM partially reversed the reduction in N-linked and O-linked occupancy of erythropoietin caused by A23187, a compound which depletes divalent cations (Kaufman et al. 1994 Biochemistry 33(33):9813-9).
rHuEPO has previously been shown to contain three N-linked and one O-linked branched carbohydrate structures that are highly sialylated (Takeuchi et al. 1988 J. Biol. Chem. 263(8):3657-3663). De-sialylated EPO is virtually inactive to induce erythropoiesis in vivo due to the rapid clearance of this modified protein by the hepatocyte asialo glycoprotein receptor (Ashwell and Harford 1982 Annual Review of Biochemistry 51(1):531-554; Goochee et al. 1991 Bio/Technology. 9(12):1347-55). Other studies have shown that sialylation and glycosylation decreases binding kinetics of EPO to the EPO receptor. (Darling et al. 2002 Biochemistry 41(49):14524-31.)
Darbepoetin alfa is a novel glycosylation analog of recombinant human erythropoietin (rHuEPO) that contains two additional N-linked glycosylation sites. Darbepoetin has decreased receptor-binding activity but exhibits a three-fold longer serum half-life and increased in vivo activity as a result of this increased persistence in circulation. The in vivo activity of EPO analogs has been demonstrated to correlate with the number of N-linked carbohydrates. (Elliott et al., Exp Hematol. 2004 32(12):1146-55.)
rHuEPO produced in CHO cells can exhibit a variable extent of glycosylation and sialylation. (Takeuchi et al., 1989 PNAS 86(20):7819-22, Zanette et al., 2003 Journal of Biotechnology 101(3):275-287). Given that EPO sialylation is an important factor in in vivo bioactivity, consistency in glycosylation and higher levels of sialylation of rHuEPO and its analogs are desirable qualities when producing recombinant protein for therapeutic uses. Thus, there exists a need for culture media and culturing methods that improve the glycosylation or sialylation of glycoproteins produced in cell cultures.